1. Field of the Invention
This invention relates generally to maximizing radio frequency transmission power and providing a universal transmitter architecture in a wireless communication device transmitter. More particularly, the invention relates to a mirror translation loop transmitter architecture operable with both constant and non-constant radio frequency (RF) envelope modulation methodologies.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards have evolved to which these systems must conform. For example, in the United States, third generation portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset π/4 differential quadrature phase shift keying (π/4-DQPSK) or variations thereof and the access format is TDMA.
Other standards may require the use of, for example, code division multiple access (CDMA). Further, new standards such as enhanced data rates for GSM evolution (EDGE), which is an extension to the global system for mobile communications (GSM to be described below) standard, and wide band code division multiple access (WCDMA) are continually emerging. One aspect that all these systems have in common is that they all use a signal having a time varying amplitude. This is sometimes referred to as a “non-constant envelope modulation.”
In Europe, the global system for mobile communications (GSM) standard requires the use of the Gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology.
Furthermore, in a typical GSM mobile communication system using narrow band TDMA technology, a GMSK modulation scheme supplies a very low noise phase-modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a highly efficient non-linear power amplifier can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is typically supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized.
Many non-constant envelope transmit architectures use a modulation scheme where both a PM signal and an amplitude-modulated (AM) signal are transmitted.
Standards employing these schemes increase the data rate without increasing the bandwidth of the transmitted signal. Unfortunately, even though it would be desirable to have one portable transceiver that can accommodate all of the above-mentioned transmission schemes, existing GSM modulation schemes are not easily adapted to transmit a signal that includes both a PM component and an AM component.
In the non-constant envelope modulation schemes typically used in mobile communications, the transmit output signal varies in both phase and amplitude. In constant envelope modulation, the transmit output signal is always at a constant amplitude. Emerging communication standards, such as EDGE and WCDMA will likely use a non-constant envelope modulation scheme. As the transmit architectures for these new standards are under development, it is generally desirable to have a single transmit architecture that supports as many standards as possible.
One possible manner of developing a single transmit architecture that is capable of both constant envelope and non-constant envelope modulation uses a conventional upconverter with filters inserted into the transmit chain. Such an architecture requires filters at the intermediate frequency (IF), at the radio frequency (RF) before the power amplifier, and at RF after the power amplifier. Unfortunately, a multi-standard transmit architecture would require many filters to be switched in and out of the transmit circuit, or would require separate transmit chains.
Further, when attempting to include a PM component and an AM component in a GSM type modulation system, the power amplifier's non-linearity could negatively affect the quality of the transmitted signal and introduce unrecoverable errors. Also, the transmitter's non-linearity could cause intermodulation products and cause regrowth of the transmit spectrum, thereby causing an unacceptable adjacent channel power ratio. Furthermore, while attempting to include a PM component and an AM component in a GSM type modulation system, the power control loop will tend to reject the amplitude variations present in the signal while attempting to maintain the desired output power. In such an arrangement, the power control loop tends to cancel the AM portion of the signal within its power control loop bandwidth.
Further still, in those transmission standards in which both a PM signal and an AM signal are sent to a power amplifier, unless the power amplifier is very linear, it may distort the combined transmission signal by causing undesirable AM to PM conversion. This conversion is detrimental to the transmit signal and can require the use of a costly and inefficient linear power amplifier.
With the increasing desirability of developing one worldwide portable communication standard, it would be desirable to allow a single portable transceiver to transmit a signal having either a constant or a non-constant envelope signal, while maximizing the efficiency of the power amplifier.